1. Field of the Invention
The present invention relates to a method and a device for process heat generation, and more particularly to a method wherein a process medium is heated to its final temperature in two stages by combustion products of one or more fossil fuels, preferably fuel oil and/or fuel gas.
2. Discussion of Background
Increasingly strict requirements are being imposed on process heat generators with respect to the maximum permissible emission of harmful substances. In this context the nitrogen oxides and sulphur dioxide present in the gases exhausted from the heating devices of heat generators are considered to be especially environmentally harmful. To minimize the formation of nitrogen oxides, the percent of nitrogen in the substances which take part in the combustion process should be maintained as low as possible without causing any detriment to the utilization of the fuel. Furthermore, the phase of the combustion having the best conditions for NO.sub.x formation should be kept as short as possible.
The first requirement can be fulfilled at stoichiometric conditions at which, however, high temperatures appear. These high temperatures favor the formation of nitrogen oxides. In such a case it is also difficult to obtain complete combustion. Because complete combustion requires a sufficient time, and thus a correspondingly long residence time for the fuels in the heating device of the components taking part in the combustion, the two conditions mentioned above cannot easily be reconciled.